Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble in such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating. Thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution, plasma gases, or have metal or metal composites deposited in the spaces of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a patterned substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate.
In the manufacture of patterned structures, such as wafer level packaging, electrochemical deposition of electrical interconnects has been used as the density of the interconnects increases. For example, see Solomon, Electrochemically Deposited Solder Bumps for Wafer-Level Packaging, Packaging/Assembly, Solid State Technology, pages 84-88, April 2001; disclosure of which is incorporated herein by reference. Wafer level packaging produces a chip/die/device that is ready for direct assembly onto the final substrate or final system platform. Wafer-level packaging is used for making electrical connections to an integrated circuit chip above the active circuitry and is especially important as the density of inputs and outputs (I/Os) on chips increases.
Wafer-level packaging schemes use a technique known as redistribution to connect the peripheral pads to an area array of solder bumps on the surface of the wafer. The basic sequence of wafer-level packaging with redistribution involves creating a level of interconnect that defines an under-bump pad that is connected to the peripheral bonding pad. The under-bump pad is exposed by a via in a dielectric layer. Then the entire wafer receives an under-bump metallurgy (UBM) stack that provides an electroplating seed layer on top of a diffusion barrier and adhesion layer. The plating mask is formed in photoresist that can range from about 1 μm to over 200 μm thick, but are typically 25-125 μm thick. Layers exceeding about 100 μm to about 125 μm are typically applied in double coats. The solder bump is electroplated within the via in the case when a thicker photoresist is used. The solder bump is typically electroplated above the photoresist when it is <50 μm thick (overplating or mushroom plating). The photoresist is then stripped and the UBM is etched away everywhere it is not covered by the solder bumps. Finally, the bumps may be reflowed, causing them to reform in the shape of truncated spheres.
Gold bumps, copper posts and copper wires for redistribution in wafer level packaging require a resist mold that is later electroplated to form the final metal structures in advanced interconnect technologies. The resist layers are very thick compared to the photoresists used in the IC manufacturing. Both feature size and resist thickness are typically in the range of 5 μm to 100 μm, so that high aspect ratios (resist thickness to line size) have to be patterned in the photoresist.
Devices manufactured for use as microelectromechanical machines also use very thick photoresist films to define the components of the machine.
Positive-acting photoresists comprising novolak resins and quinone-diazide compounds as photoactive compounds are well known in the art. Novolak resins are typically produced by condensing formaldehyde and one or more multi-substituted phenols, in the presence of an acid catalyst, such as oxalic acid. Photoactive compounds are generally obtained by reacting multihydroxyphenolic compounds with naphthoquinone diazide acids or their derivatives. Novolaks may also be reacted with quinine diazides and combined with a polymer.
Additives, such as surfactants, are often added to a photoresist composition to improve the coating uniformity of the photoresist film where the film thickness is less than 5 microns, especially to remove striations within the film. Various types of surfactants are added typically at levels ranging from about 5 ppm to about 200 ppm. U.S. Pat. No. 6,159,656 discloses the use of mixtures of surfactants from 0.01 to 2 parts by weight for a deep uv chemically amplified photoresist. These photoresists are used for high resolution deep uv lithography, where typically the film thickness of the photoresist film is less than 1 micron and surfactant levels greater than 100 ppm lead to surface defects, such as voids in the film. JP 95,021,626B2 discloses a novolak based photoresist containing a fluoro-chemical surfactant at 5 ppm.
Certain applications of the electronic industry, as discussed above, require photoresist films that are thicker than those used for high resolution lithography, where these thick films can typically be greater than 20 microns. In thick film applications the physical parameters of coating, baking and development can be quite different from those of thin films. Thick coatings of photoresist can exhibit defects such as, phase separation, striations, formation of microbubbles, etc. which can lead to nonuniform films after coating and also after development. The applicants have found that for photoresists, especially those comprising novolaks and quinone diazides, when these photoresists are coated at film thickness greater than 20 microns, these photoresists exhibit delay time instability. Delay time instability occurs when the thick photoresist films are coated and baked and then left for some extended period of time before they are exposed to irradiation. It has been observed that upon exposure and development of the photoresist film, the energy required to obtain an image increases with increasing delay time between bake and exposure. Delay time instability negatively impacts the device yield of the lithographic process and can lead to high cost of manufacture for these devices.
The applicants of the present invention have unexpectedly found that the surfactant levels that are used for thin film do not give satisfactory coating qualities when used at the same level for thick films; for example, a photoresist manufactured at higher viscosity to coat thicker films than a normal photoresist and containing only 100 ppm of surfactant, exhibits delay time instability. However, it has been further found by the applicants that there is an optimum range of surfactant levels in thick films that provide films with good coating qualities and good lithographic performance.
It is the object of the present invention to provide for a photoresist useful for coating thick films that is free of coating defects, such as striations and voids, and gives good lithographic performance, especially, providing a photoresist that is not sensitive to delay times between baking and exposure.